The invention relates to a metering device for a hot chamber die-casting machine, where the metering device includes a casting container attachable to a crucible of the hot chamber die-casting machine and having a riser channel in a riser channel area and a casting piston unit for metered conveying of melt out of the crucible via the riser channel, and a heating device with a flameless heating unit for active heating of at least a part of the riser channel area.
In the hot chamber casting process, the casting container and a casting piston of the casting piston unit are inside the liquid casting material melted in the crucible of a corresponding melting furnace, whereby the efficiency is in general considerably higher than with the cold chamber casting process. It is, for example, used in zinc and magnesium die-casting, where magnesium as the casting material has a processing temperature of typically between around 630° C. and around 660° C. depending on the alloy.
In order to prevent cooling-down problems with the stated high processing temperatures, for example in magnesium die-casting, it is known for hot chamber die-casting machines to actively heat the casting container and a nozzle that is usually attached thereto and that leads to a mould. An earlier proposal provides in this respect for gas heating of the nozzle and of the casting container at least in a connection area to which the nozzle is attached. This open gas flame heater is, however, problematic for safety reasons alone. In addition, it is difficult to heat the nozzle with a constant temperature using this technique, which can lead to nozzle deformations, and the expensive material of the nozzle and the casting container is put under relatively heavy strain by the gas flame heater.
Various alternatives to gas flame heating have, therefore, already been proposed, in particular electric resistance heaters and electric induction heaters. For example, the German laid-open publication DE 21 41 551 describes a direct electric resistance heater of a riser channel and of an adjacent nozzle, in which the riser channel and the nozzle are formed by a metallic riser channel pipe or nozzle pipe which themselves act as resistance heating elements and are surrounded by a heat-insulating material. This, however, has the drawback that the conveyed molten material is, in general, also electrically conducting and hence the heat input by the electric heater greatly fluctuates depending on the degree to which the melt fills the riser channel pipe and the nozzle pipe, so that controlled air cooling of the nozzle is provided there to prevent overheating.
In a hot chamber die-casting machine disclosed in the laid-open publication DE 24 25 067 A1, the metering device with casting container and nozzle is located completely outside the crucible, into which a filling chamber is inserted with which the metering device is connected via an associated connecting riser pipe. The filling chamber may be closed off from the crucible using a valve. By introducing an inert gas under pressure, the melt is conveyed via the connecting riser pipe into the casting container. The casting container, the nozzle, that part of the connecting riser pipe which is outside the crucible, and an overflow pipe leading from the casting container back into the crucible, are heatable by an enclosing electric induction heater.
The patent publication EP 0 761 345 B1 describes a further hot chamber die-casting machine with a generic metering device. In the arrangement therein, an inductive heating device for the nozzle and for a connection area of the casting container is provided, the inductors of which include externally insulated pipes which can be subjected to medium frequency or to a frequency around the lower high-frequency limit and through which air can flow. There, the casting container is inserted from above with the aid of a cover into the crucible, i.e. it is located with a lower part inside the crucible and with a top part containing the casting piston drive and the connection area for the nozzle outside the crucible. To permit heating of the casting container as close as possible above the crucible, the inductive heating device optionally contains an additional annular inductor placed around the casting container neck directly above the crucible cover. For forced cooling of the induction heater, an air cooling system is used instead of water cooling, which is safety-critical for example in magnesium die-casting. To do so, the inductors require sufficient installation space that cannot be reduced at will. A further problem with heating devices of the inductive type is the occurrence of stray fields, which can lead to unwelcome heating-up of other adjacent components, for example areas of the mould in the vicinity of the heated nozzle.
The technical problem underlying the invention is to provide a metering device of the type mentioned at the outset, by which the mentioned difficulties of the prior art are reduced or eliminated and which permits, in particular, reliable and safe heating of the casting container in the riser channel area outside the melting bath in the crucible using a heating device that may have a comparatively small construction.
The invention solves this problem by providing a metering device for a hot chamber die-casting machine, including a casting container attachable to a crucible of the hot chamber die-casting machine and having a riser channel in a riser channel area and a casting piston unit for metered conveying of melt out of the crucible via the riser channel, and a heating device with a flameless heating unit for active heating of at least a part of the riser channel area. The heating unit is placed in either a piston rod lead through bore through which a piston rod of the casting piston is passed, containing the riser channel and electrically insulated from the riser channel, in a riser bore or in a heater receiving space specially provided in the casting container. With this metering device, the heating device includes a flameless heating unit placed (i) inside a piston rod leadthrough bore through which a piston rod of the casting piston unit is passed, (ii) electrically insulated from the riser channel in a riser channel bore containing the riser channel, or (iii) in a heater receiving space specially provided in the casting container. The term “bore” must here be generally understood as an aperture of any cross-section, not necessarily circular.
The use of a flameless heating unit avoids the difficulties of heater types having a naked flame. The positioning locations in accordance with the invention for the heating unit permit an internal and active heating of at least a part of the riser channel area of the casting container that contains the riser channel. This permits, compared with a heater that is only on the outside, an effective and even heating of the riser channel if required from the height of the bath level, i.e. filling level, of the melting bath inside the crucible, or slightly above it. In a first positioning variant, the piston rod leadthrough bore provided in any case for passing through the casting piston rod is used, and in this case receives the heating unit. Since the piston rod leadthrough bore extends through the casting container to underneath the bath level, the heating unit may be arranged at any required depth inside the casting container. This can, in the case of a system type in which the casting container is inserted from above into the crucible so that a lower part is inside the crucible and a top part with casting piston drive and nozzle connection area is outside the crucible, preferably be a depth up to about the crucible cover or up to a normal or maximum bath level of the melt inside the crucible.
In a second positioning variant, the heating unit is inserted into the riser channel bore forming the riser channel, where it is electrically insulated from the typically metallic melt conveyed in the riser channel. This prevents fluctuations in the heating capacity when an electric resistance heating unit is selected as the heating unit. In this case too, the heating unit may be positioned at any height relative to the bath level of the melt inside the crucible.
In a third positioning variant, the heating unit is located inside a heater receiving space additionally provided for this purpose in the casting container. The height and lateral position of the latter may be selected such that the inserted heating unit heats the riser channel effectively and evenly in the required manner, in particular at or just above the melting bath level. To do so, the heater receiving space can extend, for example, at a slight distance from the riser channel and parallel or angled thereto as far as a required depth, e.g. in the case of the type with the casting container inserted into the crucible from above up to the normal or maximum bath level of the melt inside the crucible, or up to about the top edge of the crucible or to the height of a crucible cover.
In a particularly advantageous embodiment of the invention, the heating unit is an electric resistance heating unit. An electric resistance heating unit of this type can, if required, be built relatively small, i.e. it requires relatively little installation space thus permitting a particularly compact design of the metering device. The heating capacity of the electric resistance heating unit can be selectively controlled such that overheating is avoided without the absolute need for cooling ducts, which require a considerable space requirement.
In a further embodiment, the electric resistance heating unit is of a hollow-cylinder shape with a heating cylinder that has on its cylinder casing an electric heating conductor structure and is coaxially inserted into the appropriate bore or receiving space, which is designed therein as a heater bore. A resistance heating unit of this type can, firstly, be achieved at relatively low expense and, secondly, permits required, effective and constant riser channel heating. To do so, the electric heating conductor structure can be designed flexibly and suitably, for example for different heating capacities in various sections due to a correspondingly different density in the arrangement of the heating conductors and/or due to heating conductor sections with different heating conductor cross-sections. If required, the heating conductor structure may contain one or more separately controllable heating circuits. In operation, the heating cylinder can, due to the thermal expansion generally occurring, be in firm contact with or press against the adjacent bore inner wall, which contributes to its firm positioning and ensures, particularly in cases with heat transfer radially outwards, to a good heat transmission to the adjacent casting container area.
In a further embodiment, the cylinder casing of the heating cylinder contains a heat-conducting support sleeve which supports the heating conductor structure in an electrically insulating manner. The heat generated by the heating conductor structure is in this way transferred to the support sleeve and injected by the latter in an even distribution into the adjacent casting container area or riser channel area. In a further embodiment, the support sleeve is provided with thermal insulation on its inner or outer side, which improves the heat transfer into the adjacent casting container or riser channel area on the respective other side of the sleeve facing away from the thermal insulation. In addition, undesirable high temperatures on the thermally insulated side can be reliably prevented. For example, undesirable high temperatures in the piston rod leadthrough bore and for the passed-through casting piston rod, when the heating unit has been inserted into the piston rod leadthrough bore, are prevented by an internal thermal insulation of the support sleeve. In a further embodiment, an insulating sleeve made of thermally insulating material abuts against the support sleeve as a thermal insulation to form a hollow insulation space, e.g. in the form of air cushions.
A further embodiment of the invention relates to a system type where the casting container, when attached to the crucible, is inside the crucible with a crucible-side part and outside the crucible with a top part, e.g. by inserting or mounting the metering device into or onto the crucible from above. The heating cylinder extends in this embodiment of the invention in the top part as far as the crucible-side part of the casting container or at least partially inside the crucible-side casting container part. Additionally or alternatively, the heating cylinder extends in the top part of the casting container on its side facing away from the crucible at least up to the maximum height distance of the riser channel from the crucible-side part of the casting container, i.e. it extends at least as far as the riser channel away from the crucible. The latter contributes to active heating of the riser channel in its section further away from the crucible as far as the opening into the attached nozzle, while the former permits riser channel heating at or just above the bath level of the melt inside the crucible.
In an advantageously designed embodiment of the invention, the bore receiving the heating cylinder is of a conical form, and the heating cylinder is inserted with the aid of an exteriorly conical shaped adapter sleeve, on the inside of which it is arranged, into the appropriate bore. The conical shape facilitates the removal of the adapter sleeve with the heating cylinder from the bore for maintenance or replacement purposes. In a further embodiment, the tapered bore is formed by an internally tapered insertion sleeve that is of cylindrical form on the outside and that is inserted with close fit into a cylindrical receiving bore of the casting container. In this way, the casting container itself does not need to be produced with a conical bore; it is sufficient to provide the cylindrical receiving bore using simpler production technology.
In a further advantageous embodiment of the invention, the heating device contains several flameless heating units, of which one each is placed in the piston rod leadthrough bore and/or the riser channel bore and/or one or more heater receiving spaces provided specially in the casting container. Placement in this way of several heating units at various points inside the casting container with thermal contact to the riser channel can improve the evenness of the heating of the riser channel area of the casting container and reduce the temperature gradients in the heated casting container area. If required, it is also possible to place several heating units in one of the bores or heater receiving spaces at various points along the riser channel area to be heated of the casting container. It goes without saying that some or all of these heating units may each be formed by an electric resistance heating unit, for example in the form of the heating cylinder mentioned.
In another embodiment of the invention, the heating device includes a further flameless heating unit with which a nozzle connection area of the casting container and/or a nozzle attachable thereto can be additionally heated from the outside. In this case too, an electric resistance heating unit in the form of a heating cylinder laid around the connection area and/or the nozzle with the electric heating conductor structure can be used. This favors a compact design of the connection area and of the nozzle, since overheating can be prevented by suitable control of the electric heating capacity and hence voluminous cooling ducts may be dispensed with.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.